BIOMECHANICS OF FILTER FEEDING IN THE ANTARCTIC KRILL EUPHAUSIA SUPERBA: REVIEW OF PAST WORK AND NEW OBSERVATIONS

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ABSTRACT Recent observations of "compression filtration" in live euphausiids and unresolved historic controversies about filter feeding in euphausiids based mostly on inferences from preserved specimens suggest that reconsideration of the biomechanics of filter feeding is in order. Explanations of filter feeding in reviews by Mauchline and Fisher (1969) and Mauchline (1980) are incorrect. Here I review previous work and present new observations on the morphology and mechanics of the thoracic endopodites and exopodites of Euphausia superba. Movements of the endopodites are rapid and stereotyped. As the feeding basket expands laterally a pressure gradient is created which sucks water and particles into the basket from the front. Water and particles never enter the basket from the sides, below, or behind. Once inside the feeding basket, particles are retained on the filter and water is squeezed out laterally between the setae. During expansion of the basket the exopodites act as flapper valves which inhibit lateral entry of water, but during the compression stroke the exopodites lift for approximately 100 ms and permit expulsion of water. Comb setae loosen food particles trapped on the filter setae and the tips of the filter setae brush them to the mandibular palps. When phytoplankton is scarce, krill feed steadily with metronomic sweeps of the basket. When phytoplankton is abundant, they exhibit discontinuous feeding patterns. Filter feeding by compression pumping may occur in all those euphausiids that have thoracic appendages like those of E. superba and which filter feed on phytoplankton.